What impact Flynn's testimony about the Trump team can have on the investigation remains unclear.

President Trump launched a bitter Twitter tirade Sunday, attacking the FBI’s reputation with tweet bombs but not sending in any reinforcements, like evidence.

Trump also again denied that he ever asked the agency to ditch its Moscow meddling probe.

With Trump’s former national security adviser, Michael Flynn, poised to cooperate with special counsel Robert Mueller’s Russia investigation, the President defensively stormed social media with a series of denials, and fresh attacks on former FBI Director James Comey.

Months later, Comey testified before Congress that Trump had asked him to abandon the investigation, and any probe of Flynn, who pleaded guilty Friday to lying to the FBI.

“After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters — worst in History! But fear not, we will bring it back to greatness,” Trump tweeted Sunday.

Mukasey, the former attorney general, said Flynn would've either gotten immunity or a greater charge if the testimony was stronger.

(ABC)

Comey responded with his own measured tweet, repeating a quote from his congressional testimony in June: “I want the American people to know this truth: The FBI is honest. The FBI is strong. And the FBI is, and always will be, independent.”

Also coming to the FBI’s defense was Sally Yates, an Obama administration holdover, who clashed with Trump — and was fired — over an immigration travel ban in the early days of his presidency when she was the acting U.S. attorney general.

“The FBI is in ‘tatters’? No.” Yates tweeted. “The only thing in tatters is the President’s respect for the rule of law. The dedicated men and women of the FBI deserve better.”

The back-and-forth came a day after Trump fueled obstruction of justice talk with a tweet suggesting he knew about Flynn’s lies to the FBI.

Scott said he doesn't believe Trump should pardon Flynn.

(Chip Somodevilla/Getty Images)

“I had to fire General Flynn because he lied to the Vice President and the FBI,” Trump tweeted Saturday. “He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!”

Trump’s attorney, John Dowd, told ABC on Sunday that he had crafted the “sloppy” Saturday tweet.

Sen. Lindsey Graham (R-S.C.) said the President could be endangering himself with the social media screeds.

“You tweet and comment regarding ongoing criminal investigations at your own peril. I’d be careful if I were you, Mr. President. I’d watch this,” Graham said on CBS’ “Face the Nation.”

Rep. Adam Schiff of California, the ranking Democratic member on the House intelligence committee, which is also conducting an investigation, said Flynn’s guilty plea was “very significant” because court papers demonstrate he was acting at the direction of senior members of the transition team.

Feinstein said she doesn't think Flynn was a "rogue agent" in contacting Russia.

(Carolyn Kaster/AP)

“I do believe that he will incriminate others in the administration,” Schiff said on ABC’s “This Week.” “Whether that will lead ultimately to the President, I simply don’t know.”

Another top Democrat said Sunday that an obstruction of justice case is shaping up against Trump.

“What we’re beginning to see is the putting together of a case of obstruction of justice,” Sen. Dianne Feinstein (D-Calif.), the ranking Democrat on the Senate Judiciary Committee, said of that committee’s investigation.

The most powerful evidence, she said, is Trump’s firing of Comey.

“I see it, most importantly, in what happened with the firing of Director Comey, and it is my belief that that is directly because he did not agree to lift the cloud of the Russia investigation,” Feinstein said. “That’s obstruction of justice.”